[musl] [PATCH] [RFC] trap on invalid printf formats

[Rich Felker <dalias@libc.org>]

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Inspired by a new instance of some bitrotted software using %Lu
instead of %llu, attached is a draft patch to catch such errors rather
than silently leaving missing output.

I don't know if this is a good idea to actually do (note: probably
matching changes should be made in wide printf and maybe also scanf if
so) but I'm posting it here in case anyone wants to experiment or
discuss. Note that there is no conformance distinction since invalid
format strings are undefined behavior.

Rich

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diff --git a/src/stdio/vfprintf.c b/src/stdio/vfprintf.c
index 33019ff1..c3bd4d31 100644
--- a/src/stdio/vfprintf.c
+++ b/src/stdio/vfprintf.c
@@ -10,6 +10,7 @@
 #include <inttypes.h>
 #include <math.h>
 #include <float.h>
+#include "atomic.h"
 
 /* Some useful macros */
 
@@ -652,8 +653,7 @@ static int printf_core(FILE *f, const char *fmt, va_list *ap, union arg *nl_arg,
 	return 1;
 
 inval:
-	errno = EINVAL;
-	return -1;
+	a_crash();
 overflow:
 	errno = EOVERFLOW;
 	return -1;